For the past decade, the Global Positioning System (GPS) has been used to provide precise time, frequency and position co-ordinates world-wide. Recently, equipment has become available specialising in providing extremely accurate timing information, referenced to Universal Time Co-ordinates (UTC). This feature has been used at CERN to provide time of day information for systems that have been installed in the Proton Synchrotron (PS), Super Proton Synchrotron (SPS) and the Large Electron Positron (LEP) machines. The different systems are described as well as the planned developments, particularly with respect to optical transmission and the Inter-Range Instrumentation Group IRIG-B standard, for future use in the Large Hadron Collider (LHC)

Beetham, C G

Ribes, J B

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GPS PRECISION TIMING AT CERNBeetham C.G, Ribes J-B.AbstractFor the past decade, the Global Positioning System (GPS)has been used to provide precise time, frequency andposition co-ordinates world-wide. Recently, equipmenthas become available specialising in providing extremelyaccurate timing information, referenced to UniversalTime Co-ordinates (UTC). This feature has been used atCERN to provide time of day information for systemsthat have been installed in the Proton Synchrotron (PS),Super Proton Synchrotron (SPS) and the Large ElectronPositron (LEP) machines. The different systems aredescribed as well as the planned developments,particularly with respect to optical transmission and theInter-Range Instrumentation Group IRIG-B standard, forfuture use in the Large Hadron Collider (LHC). 1 INTRODUCTIONThe Global Positioning System (GPS) is claimed to be themost accurate system for worldwide distribution ofprecise time, frequency and position ever deployed. Eachof the 24 operational satellites, plus spares, carry anensemble of on-board atomic clocks which are controlledand maintained by the US military from their control basein Colorado Springs. Monitor stations in Hawaii,Kwajalein, Ascension Is. and Diego Garcia ensure aworldwide absolute time reference, with respect to UTC,from 3 to 100 ns. depending upon technique and the goodwill of the US military. The “Interface Control Document” (Letter of Intent)was signed in 1982 by six leading US firms and the“NAVSTAR USER’S OVERVIEW” was published in1986, [1]. The system was officially declared operationalin 1995. While the NAVSTAR GPS system was financedby the US most equipment sold is now for recreationaluse which explains the significant price reductions overthe recent years. Also, the system was originallyconceived for its extreme precision in position, it is onlyrecently that affordable equipment has become availableto provide the accurate time facility.This note summarises the majority of installedoperational GPS systems at CERN. To avoid duplication,identical hardware has been used for all the installations.The equipment choice was influenced more bywidespread usage of the VME standard at CERN thanprice performance factors.2 SYSTEMS2.1 Beam Dump TriggerThe initial LEP Beam Dump Triggering system wasinstalled during the 1993 shutdown. The system enablesphysicists to dump the beam whenever they consider thattheir detectors are being damaged by beam inducedradiation. This is achieved by a hardwired connectionfrom each of the four experiments, plus the control room,to the beam dump equipment located at LEP point 5.Each connection consists of a constant current loop withan electronic switch located within each experiment.Whenever the switch is activated, either manually or bythe user’s circuitry, the current loop is opened whichcauses the beam dump kicker to be fired, thus dumpingthe beam.   However, during the 1993/1994 runs a few physicistscomplained that the beam was not being dumped untilseveral milliseconds after they had activated their switch,thus causing expensive damage to their detectors. In orderto clarify this situation it was decided to install a GPSmonitoring system.  A GPS receiver is installed wherever there is a switchand also at the beam dump. When a switch is activatedthe time of day, to the nearest microsecond, (with aresolution of 300 nanoseconds), is “frozen” by thereceiver. At LEP point 5, a signal that is derived from theactual high voltage pulse, that fires the beam dumpkicker, is used to “freeze” the time in the GPS receiverlocated there. After the beam has been dumped thecontrol system reads and also “unfreezes” the GPSreceivers concerned and displays the values via the alarmsystem. Since the system became operational there hasnot been a single complaint about the beam beingdumped late.0-7803-5573-3/99/$10.00@1999 IEEE. 765Proceedings of the 1999 Particle Accelerator Conference, New York, 19992.2 Time of day source for Master TimingGenerators (MTG’s)The MTG’s are used to generate millisecond basedmachine timing for the PS, SPS and LEP machines. Theoriginal PC based systems used a “Computime”instrument made by Patek Philippe of Geneva. This unitreceives 77.5kHz radio signals from Mainflingen,Germany, and then passes them on to the MTG by meansof an RS-232 port. This resulted in a time accuracy of +-10 milliseconds.   When it was decided to upgrade the control system byreplacing the PC based 386/486 systems running XENIXwith VME platforms  using LynxOS operating systems, itwas logical to replace the Patek Philippe with VME GPS.The time of day information is now transferred via theVME backplane with an accuracy of less than twomicroseconds.2.3 CosmicsBesides monitoring the results of e+e- collisions in LEP,the four experiments are also excellent devices fordetecting cosmic particles. For many years physicistshave been exploiting this feature, either interleaved withdaily LEP operations or else during the start-up periods.Then they have access to the detectors without thebackground noise of LEP.This CosmoLep project requires the synchronisation ofthe individual observations from the four experiments.This will be achieved by tagging data with the GPS time.Each experiment will have a “time of day” generatorsynchronised to the GPS receivers located in each pit.The project may be extended to include other Europeanand possibly worldwide, laboratories.2.4 Bunch length InterlocksHigh intensity short bunches excite higher order modes inLEP. This causes overheating, exceeding the capabilitiesof the cryogenic system, resulting in equipment damage.It is proposed to install an instrument that willcontinuously monitor the bunch length and intensity ofthe beam on a turn-by-turn basis. If the set threshold isexceeded then the beam will be dumped, via thetriggering system and tagged by GPS.2.5 ATM TestsATM (Asynchronous Transfer Mode) networkingtechnology is being evaluated for its use in transportingdata in the future Large Hadron Collider (LHC). In orderto gain experience with ATM, it is being used in afeedback loop for control of betatron tunes in the SPS.The two principle elements of the Q-loop are a transversedamper, used to excite the beam, situated in building BA2and the focussing and de-focussing quadrupole magnetspowered from building BA3. Correction data is calculatedfrom beam response observed in BA2 and sent over anATM link to the power converter in BA3. The data is transmitted using a standard 53 byte ATMcell. Immediately before transmitting the cell the time ofday is read from the GPS receiver, located in BA2, andincluded in the cell. When the cell is received in BA3 alocal GPS receiver is read. The difference between thetwo times, minus the time it takes to read out a GPSreceiver, is used to calculate the latency of the system.2.6 RF InterlocksThe RF interlock system monitors up to 24 differentparameters, i.e. temperature, water flow, vacuum etc.These parameters are then multiplexed down to a singleentity that controls a switch. Each switch is included inseries with a  RF current loop circuit, which follows theLEP circumference, and any switch that is opened breaksthe current loop. This action activates the “freeze”register in the GPS receiver located at LEP point 4(ALEPH) and dumps the beam. The control system theninterrogates the GPS module and informs the alarmsystem who was responsible for the dump.   For the high intensity 1999 run a duplicate system hasbeen installed in LEP point 6 (OPAL).2.7 WorldFIPWorldFIP [2] is a fieldbus network protocol designed toprovide links between low level sensors and actuators andthe bus controllers, VME, PC etc. One of its principlecharacteristics is that it is deterministic. This feature willbe utilised to control the 1700 magnet power convertersthat will be required for LHC [3]. Although the WorldFIP macro-cycles aredeterministic, they are referenced to an internal oscillatorcontained in each bus controller. Whilst this is acceptablefor most industrial applications the inherent “drift” ofeach oscillator exceeds the LHC requirements. To overcome this problem, it is intended to externallysynchronise the WorldFIP macro-cycles to the GPSsystem. A test system is currently being assembled toevaluate if this proposal is feasible. The aim being tosynchronise the entire 1700 LHC magnet powerconverters to the GPS.2.8 Lab TestsThe lab systems are used for both hardware and softwaredevelopments. These tests resulted in the choice of theactive antenna and the type of cable to be used, inaddition to the in-line amplifier and the splitters. The labhas direct optical links, both single-mode and multimode,to the control room where one can branch into the mainCERNwide optical transmission infrastructure. Theaccurate map reference co-ordinates of the lab plus all theGPS installations were provided by CERN’s survey group766Proceedings of the 1999 Particle Accelerator Conference, New York, 1999[4].  This was done to verify the correct operation of theGPS equipment.3 EQUIPMENT3.1 ReceiverThe choice of the GPS receiver was dictated by theoperation of the LEP Beam Dump Triggering system.There were two essential requirements, the unit must becompatible with the existing SL control system and it hadto have an externally triggered hardware “freeze” register.The chosen module, TrueTime Model GPS-VME, fullyconforms to these requirements.The specifications of the module can be found atwww.trutime.com3.2 Active Antenna   The antenna used is NovAtel Model 521. It isenvironmentally sealed for protection against rain, ice andlightning strikes. For extra durability it is housed in awaterproof “Teflon” structure which protects the TNCconnector as well as the antenna.Full specifications are available at www.novatel.ca3.3 CableLow-loss coaxial cable type CERN CK50.3.4 Ancillaries4 way splitter, SCHWAIGER type VTF 7844; 5-2250MHz. 2 way splitter, Radio Materiel type VTF 7842; 5-2300MHz. In-line amplifier, Axing type SVS 2-00; 12-26dB, 450-2400MHz.4 DEVELOPMENTS4.1 OpticalFor operation of the LHC magnet power converters, thereis a requirement to transmit time of day timinginformation, referenced to the GPS standard, to each ofthe LHC alcoves. At the moment GPS referenced timingis only available in the experimental pits. The distancefrom the pits to the alcoves is approximately 1000m.   Tests have been carried out using single-mode opticalfibres in conjunction with FOXCOM optical transmitterreceivers, series 7000 [5]. It has been demonstrated thatthe GPS antenna L1 frequency at 1.575GHz, can betransmitted over a distance of 1762m. This is more thanadequate to transmit the antenna signals to each alcove.However, this solution is expensive, the transmitterreceiver units cost over 5000 dollars a pair. A cheaperalternative may be possible with the use of IRIG-B.Also being evaluated is the transmission of the currentMTG type timing over multimode fibres, usingFOXCOM type 105 transmitter/receiver units. Thesemodules are far less expensive than singlemodeequipment and transmission tests of up to 10Km havebeen successful.4.2 IRIG-BThe task of standardising instrumentation timing systemswas assigned to the Tele-Communications WorkingGroup (TCWG) of the Inter-Range InstrumentationGroup (IRIG) in 1956. The standard defines thecharacteristics of six serial time codes presently used byU.S Government agencies and private industry [6].The GPS receivers used at CERN generate a 1kHzamplitude modulated IRIG-B coded signal synchronisedto the GPS time. This will be connected to the moduleVME-SG2, also made by TrueTime. This is a time of daygenerator, either free running or synchronised by anIRIG-B input signal. The manufacturers claim anaccuracy of 1 microsecond, this will be evaluated withrespect to the GPS receiver time whilst using aninterconnecting cable of 1km. This will be done inconjunction with the WorldFIP jitter tests. This scenariomay result in a simple inexpensive method of providingGPS synchronised timing in the alcoves for the LHC era.5 CONCLUSIONSAt CERN, all the GPS systems are used to provide sub-microsecond time of day information. Since the original1993 project, the LEP Beam Dump Triggering System,GPS has been integrated into many different systems.This trend will certainly continue.For LHC operation greater use of GPS is foreseen. Thequench protection system alone will generate over 4000possible inputs to the beam dump trigger system. In orderto perform meaningful post mortem analysis, all systemswill have to be time stamped. Due to the uniformdistribution of the major systems throughout the LHCcomplex, it will be essential to use GPS as the source ofthe time reference.6 ACKNOWLEDGEMENTSTo Robin Lauckner for his support, i.e. funding, of theproject plus his assistance in testing the equipment.REFERENCES[1] GPS NAVSTAR YEE-82-009B[2] European Standard WorldFIP EN50170[3] Developments in the High Precision Control ofMagnet Currents for LHC. I.Barnett, D.Hundzinger, Q. King, J.G.Pett. This conference[4] Système Tridimensionnel de Coordonnées Utilise AuCERN. J. Gervaise, M.Mayoud, E.Menant. Copyright CERN 1976[5] FOXCOM, Website  WWW.foxcom.com[6] IRIG Serial Time Code Formats.IRIG Standard 200-98, May 1998767Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

GPS Precision Timing at CERN

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